Multi-Mode Pneumatic Artificial Muscles Driven by Hybrid Positive-Negative Pressure

Abstract

Artificial muscles embody human aspirations for engineering lifelike robotic movements. This paper introduces an architecture for Inflatable Fluid-Driven Origami-Inspired Artificial Muscles (IN-FOAMs). A typical IN-FOAM consists of an inflatable skeleton enclosed within an outer skin, which can be driven using a combination of positive and negative pressures (e.g., compressed air and vacuum). IN-FOAMs are manufactured using low-cost heat-sealable sheet materials through heat-pressing and heat-sealing processes. Thus, they can be ultra-thin when not actuated, making them flexible, lightweight, and portable. The skeleton patterns are programmable, enabling a variety of motions, including contracting, bending, twisting, and rotating, based on specific skeleton designs. We conducted comprehensive experimental, theoretical, and numerical studies to investigate IN-FOAM's basic mechanical behavior and properties. The results show that IN-FOAM's output force and contraction can be tuned through multiple operation modes with the applied hybrid positive-negative pressure. Additionally, we propose multilayer skeleton structures to enhance the contraction ratio further, and we demonstrate a multi-channel skeleton approach that allows the integration of multiple motion modes into a single IN-FOAM. These findings indicate that IN-FOAMs hold great potential for future applications in flexible wearable devices and compact soft robotic systems.

Figures (17)

• Figure 1: Design and fabrication of IN-FOAMs. (a) A linear IN-FOAM made of TPU-coated nylon fabrics. (b) Materials used to fabricate the actuator. The sheet materials have been patterned through engraving. (c) Working principle. The actuator is flat when not actuated. Skeleton inflation induces tension in the sheets to generate contraction, and voids appear between skeleton columns. After applying a vacuum to the voids, the difference in pressure relative to the atmosphere induces tension in the skin and drives the actuator to contract further. (d) A pneumatic circuit design of IN-FOAMs. (e) Fabrication process. IN-FOAMs can be fabricated rapidly in four steps: (step 1) pattern the sheets, (step 2) stack and heat-press the sheets, (step 3) cut and heat-seal layers of the skeleton, (step 4) add pneumatic connectors and heat-seal the skin.
• Figure 2: Various motions performed by IN-FOAMs. The actuators' skeletons are made of TPU-coated nylon fabrics (thickness: 0.09 mm), and the skins are made of transparent TPU film (thickness: 0.1 mm). Without actuation, the actuators are flat. (a) A linear IN-FOAM with a parallel-pouch skeleton (size: $165\times60\times0.65\: \rm{mm^3}$, in the initial state; weight: 6.26 g) contracts 17 mm driven by positive pressure and further contracts 7 mm under the negative-pressure actuation. (b) A parallel-pouch skeleton with an inflated beam at its right side enables the actuator (size: $145\times60\times0.65\: \rm{mm^3}$, in the initial state; weight: 4.31 g) to bend 47° leftwards driven by positive pressure and further bend 8° under the negative-pressure actuation. (c) With inflated beams to constrain contraction of the bottom skeleton, the actuator (size: $145\times60\times0.65\: \rm{mm^3}$, in the initial state; weight: 5.97 g) with a parallel-pouch skeleton is allowed to bend 39° upwards and further bend 23° under positive and negative pressure actuation, respectively. (d) By an anti-symmetric arrangement of oblique parallel pouches in the upper and bottom skeleton, the actuator (size: $215\times60\times0.65\: \rm{mm^3}$, in the initial state; weight: 6.65 g) twists 79° and further twists 13° under the positive and negative pressure actuation, respectively. (e) A flasher-origami-like skeleton makes the actuator (size: $160\times112\times0.65\: \rm{mm^3}$, in the initial state; weight: 10.34 g) rotate nearly 23° driven by positive pressure and further rotate 10° under the negative-pressure actuation. (f) By patterning the skeleton with L-shaped units, the actuator (size: $380\times260\times0.65\: \rm{mm^3}$, in the initial state; weight: 58.80 g) generates a length decrease of 36 mm and a width decrease of 41 mm under the positive pressure actuation; under the negative pressure actuation, the length and the width further decrease by 34 mm and 13 mm, respectively.
• Figure 3: Illustration of the modeling for the linear IN-FOAM. Cross-sectional Views of the FE simulation results are shown in Fig. \ref{['figSimulation']}(a-d). (a) Model A: the skin between skeleton columns has no contact with the skeleton under the heat-pressed boundary (denoted by point A). (b) Model B: the skin contacts with the skeleton under point A as the linear IN-FOAM contracts further. (c) Model C: when the vacuum is high enough, the upper and bottom skin between skeleton columns come into contact on the basis of Model A. (d) Model D: when the vacuum is high enough, the upper and bottom skin between skeleton columns come into contact on the basis of Model B.
• Figure 4: Static property of the linear IN-FOAM with constant positive and negative pressures. (c-f) Experimental, theoretical, and simulation results of blocked force and force-contraction. Exp.: experiment; FE: finite element simulation. In the theoretical model, the elastic modulus $E$ is set to 400 MPa, which is roughly in accord with the material's tensile test results. The factor $\delta$ (4 %) is estimated by measuring the change in a sample's length before and after the heat-pressing process. (a) The tested sample (size: $31.5\times10.6\times0.08 \:\rm{cm^3}$, in the initial state; weight: 24.6 g; skeleton length: 20 cm) at the maximum contraction ( 43 %, when $\Delta P_1=+90$ kPa, $\Delta P_2=-60$ kPa). (b) The simulated deformation at the maximum contraction, when $\Delta P_1=+90$ kPa, $\Delta P_2=-60$ kPa. The contour depicts von Mises stress (S. Mises), which concentrates on the skeleton's edges. (c) Blocked force as a function of the negative pressure $\Delta P_2$ under different positive pressures $\Delta P_1$. (d-f) Force-contraction relations when $\Delta P_1$ equals +30 kPa, +60 kPa, and +90 kPa, respectively.
• Figure 5: Two analytical approaches to the output force. (a) Approach I: cut the actuator along the center line of the void. The pushing force generated by $(P_0-P_2)$ functions as the drive force for contraction. (b) Tension force $2T_2$ as a function of contraction ratio. (c) Pushing force $(P_0-P_2)HW$ as a function of contraction ratio. (d) Approach II: cut the actuator along the center line of the skeleton pouch. The pushing force generated by $(P_1-P_0)$ functions as resistance to contraction. (e) Tension force $2T_1+2T_3$ as a function of contraction ratio. (f) Pushing force $2(P_1-P_0)H_1W$ as a function of contraction ratio.